This invention relates generally to recovering viscous petroleum from petroleum-containing formations. Throughout the world there are several major deposits of high-viscosity crude petroleum in oil sand not recoverable in their natural state through a well by ordinary production methods. In the United States, the major concentration of such deposits is in Utah, where approximately 26 billion barrels of in-place heavy oil or tar are believed to exist. In California, the estimate of in-place heavy oil or viscous crude is 220 million barrels. One of the large deposits in the world in the Province of Alberta, Canada, representing a total estimated in-place resource of almost 1000 billion barrels. The depths of these deposits range from surface outcroppings to about 2000 feet.
To date, none of these deposits has been produced comercially by an in situ technology. Commercial mining operations in a shallow Athabasca deposit and other mining projects are proposed at the present time. There have been many in situ well-to-well pilots, all of which used some form of thermal recovery after establishing communication between injector and producer. Normally such communication has been established by introducing a pancake fracture. The displacing or drive mechanism has been steam and combustion, such as the project at Gregoire Lake, or steam and chemicals, such as the early work on Lease 13 of the Athabasca deposit. Another means of developing communication is that proposed for the Peace River project where well-to-well communication is expected to be developed by injecting steam over a period of several years into an aquifer underlying the tar sand deposit at a depth of around 1800 feet. Probably the most active in situ pilot in the oil sands has been that at Cold Lake. This project uses the huff-and-puff single-well method of steam stimulation and has been producing about 5000 barrels of viscous petroleum per day for several years from about 50 wells. This is probably a semi-commercial process, but whether it is an economical venture is still unknown.
The most difficult problem for any in situ well-to-well viscous petroleum project is establishing and maintaining communication between injector and producer wells. In shallow deposits, fracturing to the surface has occured in a number of pilots so that satisfactory drive pressure within the producing formation could not be maintained. In many cases, problems arise from healing of the fracture when the viscous petroleum that had been mobilized through the application of heat then cooled as it moved toward the producer well. The cool petroleum is essentially immobile, since its viscosity in the Athabasca deposits, for example, is on the order of 100,000 to 1 million cp at reservoir temperature.
As noted, the major problem of the economic recovery from many formations has been establishing and maintaining communication between an injection position and a recovery position in the viscous oil-containing formation. This is primarily due to the character of the formations, where fluid mobility or formation permeability may be extremely low, and in some cases, such as the Athabasca Tar Sands, vitually nil. Thus, the Athabasca Tar Sands, for example, are strip mined where the overburden is limited. In some tar sands, hydraulically fracturing has been used to establish communication between injectors and producers. This has not met with uniform success. A particularly difficult situation develops in the intermediate overburden depths, which are too deep to mine economically but not deep enough to successfully hydraulically fracture from well to well.
Heretofore, many processes have been utilized in attempting to recover viscous petroleum from viscous oil formations of the Athabasca Tar Sands type. The application of heat to such viscous petroleum formations by steam or underground combustion has been attempted. The use of slotted liners positioned in the viscous oil formation as a conduit for hot fluids has also been suggested. However, these methods have not been particularly successful because of the difficulty of establishing and maintaining communication between the injector and the producer.
In issued patents assigned to the same assignee as this application, i.e. U.S. Pat. No. 3,994,340 issued Nov. 30, 1976 to D. J. Anderson et al for "Method of Recovering Viscous Petroleum From Tar Sands" and U.S. Pat. No. 4,037,658 issued July 26, 1977 to D. J. Anderson for "Method of Recovering Petroleum From An Underground Formation", techniques have been described for recovery of viscous petroleum, such as from tar sands, by using a substantially vertical passage from the earth's surface which penetrates the tar sand and a laterally extending hole containing a flow path isolated from the tar sand for circulating a hot fluid to and from the vertical passage to develop a potential flow path within the tar sand into which a drive fluid is injected to promote movement of the petroleum to a production position.